Tag: cosmology

Why is the sky dark at night?

By Phil Plait | October 1, 2012 7:00 am

One thing educators like to say is that there are no stupid questions. I disagree, mildly: unasked questions might fit that category. Still and all, even basic questions are worth asking, and sometimes can lead to profound insight.

For example, one of my favorite questions is also one of the simplest: "Why is the sky dark at night?" The wonderful folks at Minute Physics took this query on, and show you why this question is so very important.

See? Just by asking why the sky is dark, you can figure out that the Universe is neither infinite in space nor in time. It had a beginning! It doesn’t get much more profound than that.

And another thing I like about this video is that it answers this question in a very similar way to how I do. It’s something I’ve been meaning to write about here on the blog, but I suppose I waited too long. Now I don’t have to.

Funny, too. I have a BAFact I was going to post on this very topic on October 7, just a week from now! Why? Well, that’s not a stupid question at all, and I’ll answer it… in a few days.


Related Posts:

Astronomers find a galactic nursery 12.7 billion light years away
An ultradeep image that’s *full* of galaxies!
The Universe is expanding at 73.8 +/- 2.4 km/sec/megaparsec! So there.
First light, confirmed?

CATEGORIZED UNDER: Astronomy, Cool stuff

Astronomers have found when and how the cosmic fog was lifted

By Phil Plait | October 13, 2011 10:13 am

Take a look at the image displayed here [click to redshiftenate]. Every object you see there is a galaxy, a collection of billions of stars. See that one smack dab in the middle, the little red dot? The light we see from that galaxy traveled for 12.9 billion years before reaching the ESO’s Very Large Telescope in Chile. And when astronomers analyzed the light from it, and from a handful of other, similarly distant galaxies, they were able to pin down the timing of a pivotal event in the early Universe: when the cosmic fog cleared, and the Universe became transparent.

This event is called reionization, when radiation pouring out of very young galaxies flooded the Universe and stripped electrons off of their parent hydrogen atoms. An atom like this is said to be ionized. Before this time, the hydrogen gas was neutral: every proton had an electron around it. After this: zap. Ionized. This moment for the Universe was important because it changed how light flowed through space, which affects how we see it. The critical finding here is that reionization happened about 13 billion years ago, and took less time than previously thought, about 200 million years. Not only that, the culprit behind reionization may have been found: massive stars.

OK, those are the bullet points. Now let me explain in a little more detail.


Young, hot, dense, and chaotic

Imagine the Universe as it was 13.7 billion years ago. A thick, dense soup of matter permeates space, formed in the first three minutes after the Big Bang. The Universe was expanding, too, and cooling: as it got bigger, it got less dense, so the temperature dropped. During this time, electrons and protons were whizzing around on their own. Any time an electron would try to bond with a proton to form a neutral hydrogen atom, a high-energy photon (a particle of light) would come along and knock it loose again.

During this period, the Universe was opaque. Electrons are really good at absorbing photons, so light wouldn’t get far before being sucked up by an electron. But over time, things changed. All those photons lost energy as things cooled. Eventually, they didn’t have enough energy to prevent electrons combining with protons, so once an electron got together with a proton they stuck together. Neutral hydrogen became stable. This happened all over the Universe pretty much at the same time, and is called recombination. It occurred about 376,000 years after the Big Bang.

Read More

CATEGORIZED UNDER: Astronomy, Cool stuff

Another record breaker: ultra-deep image reveals ultra-distant galaxy

By Phil Plait | January 26, 2011 11:03 am

Astronomers have just announced they have discovered what may be the most distant galaxy ever seen, smashing the previous record holder. This galaxy is at a mind-crushing distance of 13.2 billion light years from Earth, making it not just the most distant galaxy but also the most distant extant object ever detected!

Here is the object in question:

The small box shows the location of the galaxy, which is invisible by eye in the image. The zoomed region shows it in the infrared, where it glows more strongly.

[NOTE: Let me be clear up front and say that this is a candidate galaxy, since it hasn’t been confirmed using other distance determination methods. However, having read the paper I think the astronomers did an excellent job showing this is very likely to be a galaxy 13.2 giga-light years away. From here on out I’ll refer to it as if it’s real, but to be fair bear in mind there is some small chance it may turn out not to be real.]

Named UDFj-39546284, the galaxy is seen as it was just 480 million years after the Universe itself formed! The previous record holder — which was announced just last October — was 13.1 billion light years away. This new galaxy beats that by 120 million light years, a substantial amount. Mind you, these galaxies formed not long after the Big Bang, which happened 13.73 billion years ago. We think the very first galaxies started forming 200 – 300 million years after the Bang; if that’s correct then we won’t see any galaxies more than about 13.5 billion light years away. Going from 13.1 to 13.2 billion light years represents a big jump closer to that ultimate limit!

The galaxy was found in the infrared Hubble Ultra Deep Field, or HUDF, an incredible observation where Hubble pointed at one patch of sky and stared at it for 173,000 seconds: 48 solid hours! After Hubble’s Wide Field Camera 3 observed it, this supposedly blank patch of sky came alive with thousands upon thousands of distant galaxies, and in fact the last record-breaking galaxy was found in the image. The picture here shows the whole HUDF image, with the first picture at the top of this post outlined. Click it to see it in full size, and you’ll start to get an appreciation of just how freaking tough these observations are. The sky is full of faint galaxies!

This new discovery was found using what’s called the dropout technique. Read More

CATEGORIZED UNDER: Astronomy, Cool stuff

Galaxy cluster at the edge of the Universe

By Phil Plait | May 10, 2010 9:57 am

Astronomers have found the most distant galaxy cluster ever seen: the sexily-named SXDF-XCLJ0218-0510.

First, the picture, then the words:

subaru_xmm_cluster

Yikes! What’s all that then?

Okay, first, this picture is littered with stars and galaxies. The galaxies are so far away they’re hard to distinguish from the stars! The dots that have arrows pointing to them are the galaxies that are most likely part of the cluster. The ones with circles have had their distance measured and are known to be part of the cluster for sure. The contour lines represent the detection of very hot gas, which is a dead giveaway that we’re dealing with a cluster here; all big clusters have gas swirling around them that gives off X-rays; the lines are like a topographic map telling you where the (otherwise invisible) gas is in the picture.

"So what?", you might say. We’ve seen lots of clusters before. Ah, but this one is different: it’s a whopping 9.6 billion light years away.

Billion. With a B.
Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science
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